1. Field of the Invention
The present invention in the fields of molecular biology, immunology and medicine relates to a chimeric nucleic acid, preferably DNA, encoding a fusion protein and its use as a vaccine to enhance immune responses, primarily cytotoxic T lymphocyte (CTL) responses to specific antigens such as tumor or viral antigens. The fusion protein comprises an antigenic polypeptide fused to a bacterial toxin translocation protein that promotes processing via the MHC class I pathway and selective induction of immunity mediated by CD8+ antigen-specific CTL.
2. Description of the Background Art
Cytotoxic T lymphocytes (CTL) are critical effectors of antitumor responses (reviewed in Refs 1-3). Activated CTL are effector cells that mediate antitumor immunity by direct lysis of their target tumor cells or by releasing of cytokines that orchestrate immune and inflammatory responses that interfere with tumor growth or metastasis. Depletion of CD8+ CTL leads to the loss of antitumor effects of several cancer vaccines (4, 5). Therefore, the enhancement of antigen presentation through the MHC class I pathway to CD8+ T cells has been a primary focus of cancer immunotherapy.
Naked DNA vaccines have emerged recently as attractive approaches for vaccine development (reviewed in 6-11). DNA vaccines generated long-term cell-mediated immunity (reviewed in 12). In addition, DNA vaccines can generate CD8+ T cell responses in vaccinated humans (13). However, one limitation of these vaccines is their lack of potency, since the DNA vaccine vectors generally do not have the intrinsic ability to be amplified and to spread in vivo as do some replicating viral vaccine vectors. Furthermore, some tumor antigens such as human papillomavirus-16 (HPV-16) E7 (5) are weak immunogens. Therefore, there is a need in the art for strategies to enhance DNA vaccine potency, particularly for more effective cancer immunotherapy.
The present inventors and their colleagues recently demonstrated that linkage of HPV-16 E7 antigen to Mtb heat shock protein 70 (Hsp70) leads to the enhancement of DNA vaccine potency (5). (See also U.S. Ser. No. 09/501,097, filed 9 Feb. 2000; and U.S. Ser. No. 099/421,608, filed 20 Oct. 1999, from which the present application claims priority) Immunization with HSP complexes isolated from tumor or virus-infected cells induced potent anti-tumor immunity (Janetzki, S et al., 1998. J Immunother 21:269-76) or antiviral immunity (Heikema, A E et al., Immunol Lett 57:69-74). In addition, immunogenic HSP-peptide complexes could be reconstituted in vitro by mixing the peptides with HSPs (Ciupitu, A M et al., 1998. J Exp Med 187:685-91). Furthermore, HSP-based protein vaccines have been created by fusing antigens to HSPs (Suzue, K et al., 1996. J Immunol 156:873-9). The results of these investigations point to HSPs a attractive candidates for use in immunotherapy. However, prior to the present inventors' work, HSP vaccines were all peptide/protein-based vaccines or, in more recent cases, were in the form of naked DNA. To date, there have been no reports of HSPs incorporated into self-replicating RNA vaccines.